Age-hardenable martensitic stainless steels of the compositions disclosed in U.S. Pat. Nos. 2,482,096 and 2,850,380 have very useful combinations of mechanical properties and corrosion resistance. For many applications, steels of this type are machined in the solution-treated condition and then subsequently hardened by a simple age-hardening treatment at temperatures between about 850.degree. and 1150.degree. F. The primary advantage of this procedure is that components and articles can be machined close to final dimensions and then subsequently hardened without encountering excessive scaling, large changes in dimensions, or difficulty in heat treatment. However, the machinability of these present age-hardening stainless steels is marginal, particularly in the solution-treated condition, and often special and costly procedures are required with them to obtain reasonable machining rates and cutting-tool life in commercial applications.
To obtain the desired fully martensitic structure in the solution-treated condition, the chemical composition of the age-hardening stainless steels must be closely controlled to minimize or eliminate delta ferrite and to control the austenite transformation characteristics. This requires a close balance between the austenite forming elements, such as carbon, nitrogen, manganese, nickel, and copper; and the ferrite forming elements, such as chromium, molybdenum, silicon, and columbium, to control the ferrite content; and of the overall composition to control the stability of the austenite formed at higher temperatures during solution-treating.
As described in U.S. Pat. Nos. 2,850,380 and 3,574,601, it is known that the machinability of the age-hardening martensitic stainless steels can be improved by increasing the sulfur content of the steels, or by adding other elements such as selenium, tellurium, bismuth or lead, which like sulfur can improve machinability. However, sulfur and these other elements have an adverse effect on hot workability and on the mechanical properties and corrosion resistance of many product forms. In bar products, for example, the sulfides produced by the sulfur additions elongate in the direction of hot rolling, producing sulfide stringers which markedly reduce transverse impact strength and ductility, and overall mechanical properties. Also, the marked tendency of sulfur to segregate in large, conventionally cast ingot sections has a marked detrimental effect on the soundness, polishability, and texturizing properties of plastic molds produced from these materials. Therefore, with prior art steels of this type, sulfur is desirably included from the standpoint of enhancing machinability, but only at a significant sacrifice of toughness, ductility, corrosion resistance, polishability, texturizing, and other related properties.